An airplane is propelled by a number of jet engines each housed in a nacelle.
A nacelle generally has a tubular structure comprising an air intake forward of the jet engine, a middle section intended to surround a fan of the jet engine, a downstream section that may incorporate thrust-reversal means and is intended to surround the combustion chamber of the jet engine, and generally ends in a jet pipe, the outlet of which is situated downstream of the jet engine.
Modern nacelles are intended to house a bypass jet engine capable of generating, on the one hand, a flow of hot air (also known as the primary flow) from the jet engine combustion chamber and, on the other hand, a flow of cold air (the secondary or bypass flow) from the fan and flowing around the outside of the jet engine through an annular passage, also known as a flow path, formed between an inner structure defining a cowling of the jet engine and an inner wall of the nacelle. The two air flows are ejected from the jet engine via the rear of the nacelle.
The parts of the rear section which are kept fixed in flight, namely the inner structure, but also the outer structure of the rear section, are generally connected to the middle section by a clamping system of the knife-edge/groove type.
This type of clamping holds the rear section in place and is able to react longitudinal loads while at the same time permitting radial disengagement to allow the outer and inner structures to be opened easily about a hinge line situated at the 12 o'clock position so that maintenance operations can be carried out on the inside of the nacelle or on the jet engine. These are held closed in flight by latches at the six o'clock position.
Reversers and cowls are generally designed in the form of discrete structural elements, such as longitudinal beams, actuating cylinders, supporting and/or actuating continuous non-structural elements such as acoustic engine cowling panels, moving thrust reverser panels, etc. Such designs lead to the introduction of highly localized loads into certain regions of the groove/knife-edge clamps, particularly those regions situated at the beams and other structural elements.
It has been found that, on the existing structures, these peripheral knife-edge/groove clamps are ill-suited to structures that have very individualized load paths. This results in low tolerance to damage. Further, these clamps are generally made of aluminum and are simply engineered for fatigue strength rather than for damage limitation.
In addition, the clamps on the opening elements are unable to react circumferential (hoop) loads, because they are not axisymmetric.
It is difficult to conceive of making an entire clamp out of a stronger material because this would have a negative impact in terms of the cost and mass of the propulsion unit.